iris: Fix "Force Zero RTA Index Enable" setting again

In 2ca0d913ea8, we began updating cso_fb->layers to the actual layer
count, rather than 0.  This fixed cases where we were setting "Force
Zero RTA Index Enable" even when doing layered rendering.  Sadly, it
also broke the check entirely: cso_fb->layers is now 1 for non-layered
cases, but the Force Zero RTA Index check was still comparing for 0.

Fixes: 2ca0d913ea ("iris: Fix framebuffer layer count")

src/gallium/drivers/iris/iris_state.c

@@ -5509,7 +5509,7 @@ iris_upload_dirty_render_state(struct iris_context *ice,
              BRW_BARYCENTRIC_NONPERSPECTIVE_BITS)
             cl.NonPerspectiveBarycentricEnable = true;
 
-         cl.ForceZeroRTAIndexEnable = cso_fb->layers == 0;
+         cl.ForceZeroRTAIndexEnable = cso_fb->layers <= 1;
          cl.MaximumVPIndex = ice->state.num_viewports - 1;
       }
       iris_emit_merge(batch, cso_rast->clip, dynamic_clip,